Defects in organelle dynamics underlie a number of human degenerative disorders, and whole exome sequencing (WES) is a powerful tool for studying genetic changes that affect the cellular machinery. WES may uncover variants of unknown significance (VUS) that require functional validation. Previously, a pathogenic de novo variant in the middle domain of DNM1L (p.A395D) was identified in a single patient with a lethal defect of mitochondrial and peroxisomal fission. We identified two additional patients with infantile encephalopathy and partially overlapping clinical features, each with a novel VUS in the middle domain of DNM1L (p.G350R and p.E379K). To evaluate pathogenicity, we generated transgenic Drosophila expressing wild-type or variant DNM1L. We find that human wild-type DNM1L rescues the lethality as well as specific phenotypes associated with the loss of Drp1 in Drosophila. Neither the p.A395D variant nor the novel variant p.G350R rescue lethality or other phenotypes. Moreover, overexpression of p.A395D and p.G350R in Drosophila neurons, salivary gland and muscle strikingly altered peroxisomal and mitochondrial morphology. In contrast, the other novel variant (p.E379K) rescued lethality and did not affect organelle morphology, although it was associated with a subtle mitochondrial trafficking defect in an in vivo assay. Interestingly, the patient with the p.E379K variant also has a de novo VUS in pyruvate dehydrogenase 1 (PDHA1) affecting the same amino acid (G150) as another case of PDHA1 deficiency suggesting the PDHA1 variant may be pathogenic. In summary, detailed clinical evaluation and WES with functional studies in Drosophila can distinguish different functional consequences of newly-described DNM1L alleles.
Peroxisome biogenesis disorders (PBD) are a group of multi-system human diseases due to mutations in the PEX genes that are responsible for peroxisome assembly and function. These disorders lead to global defects in peroxisomal function and result in severe brain, liver, bone and kidney disease. In order to study their pathogenesis we undertook a systematic genetic and biochemical study of Drosophila pex16 and pex2 mutants. These mutants are short-lived with defects in locomotion and activity. Moreover these mutants exhibit severe morphologic and functional peroxisomal defects. Using metabolomics we uncovered defects in multiple biochemical pathways including defects outside the canonical specialized lipid pathways performed by peroxisomal enzymes. These included unanticipated changes in metabolites in glycolysis, glycogen metabolism, and the pentose phosphate pathway, carbohydrate metabolic pathways that do not utilize known peroxisomal enzymes. In addition, mutant flies are starvation sensitive and are very sensitive to glucose deprivation exhibiting dramatic shortening of lifespan and hyperactivity on low-sugar food. We use bioinformatic transcriptional profiling to examine gene co-regulation between peroxisomal genes and other metabolic pathways and we observe that the expression of peroxisomal and carbohydrate pathway genes in flies and mouse are tightly correlated. Indeed key steps in carbohydrate metabolism were found to be strongly co-regulated with peroxisomal genes in flies and mice. Moreover mice lacking peroxisomes exhibit defective carbohydrate metabolism at the same key steps in carbohydrate breakdown. Our data indicate an unexpected link between these two metabolic processes and suggest metabolism of carbohydrates could be a new therapeutic target for patients with PBD.
We present a patient with a unique neurological phenotype with a progressive neurodegenerative. An 18-year diagnostic odyssey for the patient ended when exome sequencing identified a homozygous PEX16 mutation suggesting an atypical peroxisomal biogenesis disorder (PBD). Interestingly, the patient's peroxisomal biochemical abnormalities were subtle, such that plasma very-long-chain fatty acids initially failed to provide a diagnosis. This case suggests that next-generation sequencing may be diagnostic in some atypical peroxisomal biogenesis disorders.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.